Male Mate Choice for Unparasitized Females in Drosophila melanogaster

Parasite-mediated sexual selection theory suggests that females may choose male partners who possess a relatively low pathogen load in order to maximize their reproductive investment (e.g. by avoiding pathogens or producing pathogen-resistant offspring). However, males might also maximize their reproductive investment by preferring unparasitized females; even in non-sex-role-reversed systems where female choice predominates. We address this hypothesis in the fruit fly Drosophila melanogaster where female choice plays a major selective role. Females in this system exhibit reduced fecundity when parasitized, creating the opportunity for simultaneous parasite-mediated male choice. To address our hypothesis, virgin females were either prick-injected with a sub-lethal dose of the gram-negative bacteria Pseudomonas aeuginosa (pricked-parasitized group, PP), prick-injected with sterile media (pricked-unparasitized group, PU) or unpricked and unparasitized (UU group). Twenty-four hours post-infection, females were decapitated to minimize the confounding effects of female behavior on male preference (decapitated females will stand upright, but lack reproductive behavior). Males were then assigned to one of two different two-choice trials (experiment A: PU versus UU females or experiment B: PP versus UU females) and their courtship behavior monitored. In experiment A, males exhibited no courtship preference for either female. However, experiment B males preferentially courted UU females more often than PP females. These results suggest that males prefer unparasitized mates, which could play a significant role in shaping disease resistance in this system. These data also provide some of the first evidence that males prefer unparasitized females in systems that do not exhibit sex-role reversal.